1. Field of the Invention
The present invention relates to polarization-maintaining optical fiber and more particularly to determining the angular orientation of the polarization axes within a length of such fiber.
2. Description of the Prior Art
Polarization-maintaining (PM) optical fiber generally has some physical provision for maintaining the state of linearly polarized light traveling through a single-mode core (Noda, J., Okamoto, K. and Sasaki, Y., "Polarization Maintaining Fibers and Their Applications", J. Lightwave Technology, LT.4, No. 8, pg. 1071, 1986). In most instances, this involves either the use of an elliptical core geometry or application of preferentially oriented compressive or tensile stresses surrounding a circular optical core (see FIGS. 1-4). The latter fibers have some form of stress-applying parts (SAPs) made of a glass having different thermal expansion properties than the surrounding fiber's glass matrix. By observing a cleaved or polished fiber endface under high magnification, one can readily discern the angular orientation of either the geometric core or SAP-style fibers, and hence the orientation of the principle polarization axes. Direct observation of PM fiber ends is, however, not always possible. Procedures including PM fiber splicing often require angular alignment while the fiber ends are butted together or otherwise visually inaccessible. A new means for determining the angular orientation of these polarization axes within a length of such fiber without examination of prepared endfaces is required.
The prior art of probing the orientation of polarization axes in PM optical fiber fall into only two categories known. The first consists of applying a compressive stress to the side of a PM fiber and observing a change in the fiber's polarization crosstalk or extinction ratio (Carrara, S. L. A., Kim, B. Y. and Shaw, H. J., "Elasto-optic alignment of birefringent axes in polarization-holding optical fiber", Optics Letters, 11, pg. 470, 1986). The other method is the Profile Alignment System (PAS) developed by Fujikura (Fujikura Ltd. product bulletin #88112000 on the FSM-20 PM, pg 2, 1990). A collimated whitelight source is incident to the side of a PM fiber and the resulting refracted image through the fiber is sensed via a CCD video camera and processed to resolve the apparent position of the SAP's, thus locating the polarization axes. Although the PAS method is best suited to the circular SAP geometry of Fujikura PANDA fiber (see FIG. 3), it has shown to have some success in locating polarization axes in one other type of non. circular SAP fiber (shown in FIG. 2) manufactured by Corning. The invention described herein, besides being able to locate polarization axes in these two types of SAP-containing fibers, can also locate axes in PM fiber having elliptical core regions as well.